Application of Coefficient of Anisotropy to the Geophysical Prospecting of Campo—Ma’an Area of the Ntem Complex in Cameroon

Audiomagnetotelluric data were acquired for 10 sounding stations in open fields and roads along the main highways from Campo to Ma’an in Cameroon. An application of the coefficients of anisotropy was used to determine the tectonic settings and the nature of the contacts between the sedimentary formation of the Kribi-Campo basin and the cratonic Ntem Complex. The results of this study show that apart from the sedimentary formation, the metamorphic formation in this area comprises two metamorphic blocks: a regional metamorphic block which might be igneous or granitic in nature and composition and seem to be Precambrian and plutonic in origin and a contact or transitional metamorphic block, which might represent schist or gneiss rocks. The contact or transitional metamorphic block might be as a result of a long period of contact between regional metamorphic rocks and rocks of sedimentary formation. These contact/transitional metamorphic rocks which are found sandwiched between rocks of sedimentary formation and rocks of the regional metamorphic block and at varied depths confirm the existence of vertical and sub-vertical contacts between the Kribi-Campo basin and the cratonic Ntem complex. Two types of rock contacts have been identified: a sedimentary-schist contact which is found at the boundary between the sedimentary formation and the transitional metamorphic block and a schist-granite contact between the transitional and regional metamorphic blocks. Two main formations: a sedimentary formation, which is found in the Kribi-Campo sub-basin and the Ntem River bed and its flood plains and the metamorphic formation which includes both the transitional and regional metamorphic How to cite this paper: Njingti-Nfor, Louise, O.A.M., Malquaire, K.P.R., Dieudonné, B. and Achille, L.T.J. (2017) Application of Coefficient of Anisotropy to the Geophysical Prospecting of Campo—Ma’an Area of the Ntem Complex in Cameroon. Open Journal of Geology, 7, 1801-1818. https://doi.org/10.4236/ojg.2017.712121 Received: October 28, 2017 Accepted: December 25, 2017 Published: December 28, 2017 Copyright © 2017 by authors and Scientific Research Publishing Inc. This work is licensed under the Creative Commons Attribution International License (CC BY 4.0). http://creativecommons.org/licenses/by/4.0/


Introduction
According to [1], the geometrical arrangement of interstices in rocks has a pronounced effect on their electrical conductivity. This is as a result of the fact that due to the water contained in these interstices, the conduction of electric current tends to be electrolytic rather than electronic. As a result of this, the resistivity of rocks varies with the mobility, concentration and degree of dissociation of ions in the interstices. Rocks are usually deposited in layers such that the electrical conduction varies depending on the direction of deposition. In an isotropic medium, the propagation of telluric or electric current is in all directions [2]. In cases where the conductivity depends on the direction of the current, the medium is said to be anisotropic [3]. In general, sedimentary rock formations are considered to be isotropic, however according to [1], the anisotropy of rocks and rock materials is characteristic of their stratification. This is demonstrated in the field during Magnetotelluric prospection by the fact that for a homogeneous subsurface, telluric current tends to flow in the direction of strike of the bedding planes than in the more resistive perpendicular direction. And that is why in magnetotelluric data collection the transverse and perpendicular directions of apparent resistivity must first be determined by the method of rotation [4]. The assertion by [1] implies that the geometrical arrangement of the interstices in rocks also has a pronounced effect on their anisotropy. [3] also identifies the case of micro-anisotropy in which in laminated rocks, the laminas have different resistivity values. By employing the magnetotelluric method in the field, [5] came across rocks that they considered to be laterally isotropic. For this reason, the apparent resistivity values also vary with lateral directions. The following statements enable the coefficient of anisotropy of resistivity to find its place in magnetotelluric prospecting: -According to [5], an on the spot calculation of coefficients of anisotropy can, in addition to the method of rotation by [4]; be a very useful tool in determining the direction of structural trend of the geologic features in a particular region of prospecting. of coefficient of anisotropy can be used to determine in a vivid manner the types of rocks formations in the region of investigation [5]. -From a pseudo-section of anisotropy it is possible to distinguish completely isotropic from completely anisotropic zones [5]. These "pseudo-section of anisotropy could be an important tool for an initial modelling".
-For a 2-D model made up of two adjacent blocks with quite different apparent resistivity values (fault, contact, fracture, and intrusion) it can be deduced that telluric current circulates along the strike of the more resistive block.
The coefficient of anisotropy can be used for such a 2-D structure to locate the boundary between conductive and resistive blocks, and to determine the direction chosen by telluric current near important discontinuities [6].
The revelations above show the importance of anisotropy coefficient in magneto-telluric prospecting and in the analysis and interpretation of the acquired data. The present work, which is based on these findings, has as main objective the application of the coefficients of anisotropy to determine the tectonic settings and the nature of the contacts between the sedimentary formation of the Kribi-Campo basin and the cratonic Ntem Complex in the Campo-Ma'an area of South Cameroon.

Geo-Tectonic and Geological Settings
The entire African continent underwent a major phase of basement re-activation in the Late Precambrian, which resulted in the structural differentiation of the continent into stable cratonic nuclei, surrounded by mobile belts [7] [8]. This event has controlled the subsequent tectonic history that has affected the whole continent with the most recent events being the East African rift that was initiated some 30 Ma to 40 Ma [9]. Studies of absolute plate motion show that at present, the African plate is almost stationary and this may be the reason why some of the most recent tectonic movements in the continent have been dominated by vertical and oscillatory motions which have led to either intrusions from the underlying substratum or subductions of near-surface materials into the architectural settings of subsurface formations [10] [11].
The Congo Craton of central Africa is composed of Archean crust, early to mid-Proterozoic fold belt and Proterozoic cover exposed in a ring of Archean terranes of diverse size and separated from one another by intervening Proterozoic belts or cover surrounding the Congo Basin [12]. Most of these terranes have been stable since the end of the Archean. This Craton being one of the cratonic nuclei is a large sub-circular mass of about 5.7 × 10 6 km 2 in area and has an approximate diameter of 2.5 × 10 3 km. The north western margin of the Archean Congo Craton which is found in the South of Cameroon is called the Ntem Complex [13] [14]. The Ntem Complex is divided into three main structural domains: to the north there is a major thrust that marks the contact with the Pan-African orogenic mobile belt called the Yaounde group, to the northwest end there are the Nyong and Ayina series and in the south-central area the Ntem Open Journal of Geology series [15]. The Ntem series is dominated by massive and banded plutonic rocks of the charnockite suite and by intrusive tonalites, trondhjemites and granodiorites [16]. The entire Ntem Complex in particular and the Congo Craton in general are involved in two of the three orogenic cycles that are known in Cameroon: -The Liberian cycle which dates from the Archean era (about 2.5 Ma old) and is bounded by major thrusts that separate the Ntem series from the Nyong unit to the Northwest and the Pan-African Yaounde Group to the North of the Ntem Complex [17]. This era began with the intrusion of magmatic rocks from which the greenstone belts were derived. Greenstone belt formation was followed by diapiric intrusion of the Tonalite-Throngemite Granodiorite (TTG) between 2900 Ma and 2800 Ma, during the major tectono-metamorphic phase [16]. The structures formed are essentially vertically dipping and the metamorphism dominated by granulite facies rocks that ended with an important migmatisation event, resulting in the intrusion of anatectic potassic granitoids [18].
-The Eburnean or Transamazonian cycle that dates from the Palaeoproterozoic period (2.5 Ma to 1.8 Ma ago). This corresponds to the Nyong and Ayina series found to the north western border and to the east of the Ntem complex [19]. This Eburnean orogenic cycle is characterized by intrusion of doleritic dykes; which cycle ended with either a thermal or hydrothermal event at around 1800 Ma [16].
The present study area is found at the south-western edge of the Ntem Com-

Magnetotelluric Data
The magnetotelluric data used in this work was acquired during a field study that took place in March 2015 using an ECA 540 AMT resistivity meter which has a frequency range from 4.1 Hz to 2300 Hz giving a total number of twelve

Coefficient of Anisotropy Equation
According to [5], related equations of resistivity coefficient of anisotropy are simple and easily derived when a medium is isotropic. They considered an anisotropic and tabular medium in which the values of apparent resistivity in a direction perpendicular to the structural discontinuity are higher than those parallel to it. These are called transverse apparent ( ) the resistivity and the flow of current is given according to [22] by the expression: The equation above also represents the flattening coefficient for a homogeneous but anisotropic medium where the equipotential surfaces about a point source of current are flattened ellipsoids of revolution [22]. According to [5] the derivation of the formula for the coefficient of anisotropy as above, allows for a deduction of its dependence on the geo-electric section, which may in turn depend on the geologic section (stratigraphic or lithological) for rocks covering a long geological period [23].

Pseudo Depth of Penetration of Telluric Currents
According to the principle of the skin-effect upon which the magnetotelluric method is based, the depth of penetration of telluric current decreases with an increase in frequency until this current becomes completely attenuated [24].
This enables the pseudo-depth of penetration of the telluric current to be determined using the equation: where T is period, which the reciprocal of frequency measured in seconds For each station and as a function of the frequency and the geometric mean apparent resistivity value, the pseudo-depth of penetration of the telluric current is determined.
From the two values of apparent resistivity got in the two principal directions, the geometric mean value of apparent resistivity, ρ , measured in m Ω is calculated using the formula:

Geoelectric Sections
A geo-electric section is a 2D presentation of the earth's subsurface that is described by two fundamental parameters: the resistivity of the subsoil i ρ and thickness of depth i h [25]. The geo-electric section can also be described as a function of other parameters such as conductance, resistance and coefficient of anisotropy. It is thus employed to show how the electrical properties of rocks and rock materials can be averaged over a large volume of earth, which may not necessarily be homogeneous [25]. For a layered sequence of rocks, the Open Journal of Geology geo-electric section shows boundaries between layers represented by pronounced resistivity contrasts. In the geo-electric section, it is possible to recognize structures like folds, faults and intrusions. A fault is recognized in a geo-electric section by a break in the continuity of the normal geo-electric sectional sequence [25]. Some amount of geo-electric section is always repeated or missing at the fault contact [26]. From the geo-electric section, a measure of the fault magnitude is equal to the thickness of the missing or repeated section [27].
A geo-electric section differs from a geologic section when the boundaries between geologic layers do not coincide with the boundaries between layers characterized by different resistivity values. This is because, the electric bounda-

Iso-Anisotropy Contour Maps
An iso-anisotropy contour map is a plot of the pseudo-depth of penetration of telluric current in vertical axis against the distance from a base station taken as origin on the horizontal axis. This plot gives a series of contour lines that represent the paths connecting points with the same coefficient of anisotropy values. These contour lines provide a two-dimensional representation of the variation of coefficient of anisotropy with depth for vertical cross-section through a slice of earth under consideration. They illustrate the shape of the structures within the sub-surface. According to [28], when the sub-surface is homogeneous and has uniformly varying apparent resistivity values, the con-     in this area of study might be made up principally of limestone and sandstones [28]. According to [30], this formation may also consists of conglomerates and Apart from the sedimentary formation, two types of metamorphism can be observed from the iso-anisotropy contour maps. There is regional metamorphism, which is depicted by the rocks with values of coefficient of anisotropy of 1.4 λ ≥ , which might be igneous or granitic in nature and composition and seem to be Precambrian and plutonic in origin. These rocks are found at the base in all the Figure 3 , which might represent schist or gneiss rocks. This might be as a result of a long period of contact between regional metamorphic rocks and rocks of sedimentary formation. The resultant effect could be consolidation probably due to compaction under high petro-static pressure, high temperature and multidirectional tectonic forces. Such contact/transitional metamorphic rocks are found in the Figures 3(a)-(e) sandwiched between rocks of sedimentary formation and rocks of regional metamorphic formation and at varied depth in what seems to be due to a vertical and sub-vertical contacts. According to [31], such vertical motions as witness in Figures 3(a)-(e) are also associated with the development of intracratonic sedimentary basins, which is considered as one type of cratonic deformation that has occurred throughout geologic records. In Figures 3(d) rocks of the various formations are almost horizontally layered.

Analysis and Interpretation of Iso-Anisotropy Contour Maps
Two types of rock contacts can be identified in these figures. There is the sedimentary-schist contact which is found at the boundary between the sedimentary formation and the transitional metamorphic block. And there is the schist-granite contact between the transitional and regional metamorphic blocks. It can be seen that for Figures 3(a)    extend and is found mostly at near surface, hardly exceeding 1 km in depth, except in Figure 4(a) where this material is lodged inside the high resistivity material at an approximate pseudo-depth of 20 km. This low resistivity material found within the subsurface of Figure 4(a) seems to originate from a lateral embodiment of such material from a reverse faulting system to the far end of the geo-electric section. At near surface, these low resistivity materials are mostly concentrated to the western extremes of the profiles, which are close to the contact zone between the sedimentary Kribi-Campo basin and metamorphic Ntem Complex, and explains the sedimentary nature of the materials. The moderately Open Journal of Geology Such is the case with geo-electric sections of Figure 4(a), Figure 4(c) and Figure  4(d).

Analysis and Interpretation of Geoelectric Sections
The geo-electric sections reveal that the entire area of study is mostly made up of materials of very high resistivity, , which seems to have originated from depths of ( )

Conclusions
This work had as main objective the application of the coefficients of anisotropy to determine the tectonic settings and the nature of the contacts between the sedimentary formation of the Kribi-Campo basin and the cratonic Ntem Complex in the Campo-Ma'an area of South Cameroon. The magnetotelluric data collected for 10 sounding stations and organized in 5 profiles have been used to draw 5 iso-anisotropy contour maps and construct 5 geo-electric sections. From the iso-anisotropy contour maps, it has been deduced that apart from the sedimentary formation, the metamorphic formation comprises two metamorphic blocks: a regional metamorphic block which might be igneous or granitic in nature and composition, and seems to be Precambrian and plutonic in origin and a contact or transitional metamorphic block, which might represent schist or gneiss rocks. The contact or transitional metamorphic block might be as a result of a long period of contact between regional metamorphic rocks and Open Journal of Geology rocks of sedimentary formation. Due to this long period of contact, the resultant effect could be consolidation brought about by compaction under high petro-static pressure, high temperature and multidirectional tectonic forces. In most cases, the contact/transitional metamorphic rocks are found sandwiched between rocks of sedimentary formation and rocks of regional metamorphic block and at varied depth in what seems to be due to a vertical and sub-vertical contact. Two types of rock contacts have been identified: a sedimentary-schist contact which is found at the boundary between the sedimentary formation and the transitional metamorphic block and a schist-granite contact between the transitional and regional metamorphic blocks. Also, from the iso-anisotropy contour maps, two main formations have been identified in the area of study. There is the sedimentary formation, which is found in the Kribi-Campo sub-basin and the Ntem River bed and its flood plains and the metamorphic formation which includes both the transitional and regional metamorphic rocks. This metamorphic formation of the Ntem Complex occupies a greater portion of the entire area with a complete absence of sedimentary cover in most of the localities.
From the geo-electric sections which were constructed from transverse apparent resistivity values, it has been observed that the moderately high and very high resistivity materials put together occupy almost the entire subsurface in the area of study confirming its high vertical tectonic stability and by extension that of the entire Ntem Complex.